Electric cigar lighters for automobiles, boats, jet engine igniters and like devices commonly utilize spiral-wound heating coils. The cigar lighters to which this invention is applied herein are well known, particularly in the automotive industry where most cars are equipped with one or several such devices. For many years during the development of the automobile, electric cigar lighters consisted essentially of a cylindrical receptacle and a spring-biased removable plug which was normally carried in the receptacle, the plug having a spiral, 6-volt igniting coil at its inner end. The igniting coil was mounted in a metal cup, and was heated by shifting the plug deeper in the receptacle so as to cause engagement of the metal cup with a metal or bimetallic contact clip that completed an electrical circuit through the coil. In automatic lighters, by the time the coil heated to its normal,tobacco-igniting temperature, the concurrently heated bimetallic clip had spread apart, rleasing the cup whereupon the plug snapped forward and broke the circuit. The plug was then ready for removal from the socket for use.
After years of manufacturing cars equipped with 6 or 7 volt electrical systems, the car manufacturers saw the advantages of, and adopted a 14 volt system, utilizing a 12 volt battery instead of the prevailing 6 volt size. This, however, necessitated a redesign of the cigar lighter coils, since it now became necessary to utilize a thinner wire or ribbon having a greater length, in producing the igniting coil. At the same time, the coil-carrying cups in the lighters retained their original diameters. The new coils accordingly were much less rugged, since the thinner, longer 12 volt or 14 volt ribbon lacked the support and physical strength characteristics of its 6 volt predecessor.
The change in ribbon size was especially important, since the heating coil constituted a particularly critical part of all car lighters. In use, the coil is repeatedly heated to a high degree of incandescence and then applied to the tips of the cigars and cigarettes to light the tobacco. This coil is ordinarily secured only at its respective ends, and the many spiral convolutions are otherwise unsupported except for a backing member of mica, ceramic or other material. The ribbon is made of thin gauge stock due to the need for compactness, since it is required to fit within the original 6 volt size cup or in shells of even smaller diameter. And, it is subjected to various kinds of stress, shock and abuse including elevated temperatures, vehicle vibration, pressure against cigar and cigarette tips, and tapping or scraping to remove burned tobacco particles.
Efforts have been directed toward redesigning the higher voltage heating coils so they can resist such stresses and abuse, thereby to assure more reliable performance as well as retention of the original shape and alignment despite considerable use.
It is already known that the ribbon making up the igniting coil can be advantageously grooved, formed or bent laterally to give a cross-sectional configuration or contour which provides a nesting or interlocking relationship between successive convolutions. For example, U.S. Pat. Nos. 2,959,663 and 2,959,664 granted on Nov. 8, 1960 to L. E. Fenn and U.S. Pat. No. 3,909,587 granted on Sept. 30, 1975 to Donald J. Mattis show various cross-sectional configurations of a ribbon-like coil strip intended for automatic electric cigar lighters. While these solved a number of the existing problems, they represented costly or slow procedures or else were subject to failures due to cracking or overstretching of the metal at the bends. Limitations existed as to the ribbon thicknesses which could be used, and problems were had in obtaining the necessary very high quality. Moreover, the Fenn proposals did not provide for sufficiently high overall density of heater metal in the coil while at the same time retaining low cost, since the proposals which achieved high density required special costly ribbon sections of unusual, non-uniform thickness.
The requirements of ruggedness or strength, compactness and small size of heating coils also exist in the field of jet engine igniters and kindred devices. Here, large units are especially undesirable, and existing voltage and current supplies are not easily changed. For such use, high metal density in the coil and reliability of performance coupled with low cost represent important considerations, in addition to small coil size.